Utilization of an extracting agent as antifoaming agent in the production of anhydrous formic acid

ABSTRACT

The invention relates to a process for obtaining anhydrous or substantially anhydrous formic acid in which, during the work-up, a compound of the general formula I  
                 
 
     where the radicals R1 and R2 are alkyl, cycloalkyl, aryl or aralkyl groups, or R1 and R2 jointly, together with the N atom, form a heterocyclic 5- or 6-membered ring, and only one of the radicals is an aryl group, and where R3 is hydrogen or a C1-C4-alkyl group,  
     is employed simultaneously as extractant for formic acid and as antifoam for a distillation process.

[0001] The present invention relates to an apparatus and a process forthe preparation of anhydrous or substantially anhydrous formic acid, andto the use of the extractant employed in the work-up of formic acid.

[0002] “Ullmanns Encyklopädie der technischen Chemie” [Ullmann'sEncyclopedia of Industrial Chemistry], 4th Edition, Volume 7, page 365,discloses that formic acid can be prepared by acidolysis of formamideusing sulfuric acid. However, this process has the disadvantage thatstoichiometric amounts of ammonium sulfate are obtained as anunavoidable product.

[0003] Another way of preparing formic acid consists in the hydrolysisof methyl formate, which is synthesized from methanol and carbonmonoxide. This synthesis is based on the following equations: CO +CH3-OH → CH3-O-CO-H CH3-O-CO-H + H2O → CH3-OH + H-CO-OH overall: CO +H2O → H-CO-OH

[0004] The hydrolysis of methyl formate described in “UllmannsEncyklopädie der technischen Chemie” [Ullmann's Encyclopedia ofIndustrial Chemistry], 4th Edition, Volume 7, page 366

HCOOCH₃+H₂O ⇄HCOOH+CH₃OH

[0005] has the disadvantage of an unfavorable position of the hydrolysisequilibrium. A shift in the equilibrium by removing the desired processproducts by distillation is not possible since methyl formate (boilingpoint 32° C.) boils significantly lower than methanol (boiling point 65°C.) and formic acid (boiling point 101° C.). Anhydrous formic acidcannot easily be obtained from the resultant aqueous formic acidsolution by distillation since it forms an azeotrope with water. Thedifficulty thus consists in obtaining anhydrous formic acid from themethyl formate hydrolysis mixture.

[0006] A process described in EP-B-0 017 866 which comprises steps a) tog) enables the preparation of anhydrous formic acid starting from methylformate. Anhydrous formic acid is obtained here if

[0007] a) methyl formate is subjected to hydrolysis,

[0008] b) methanol and excess methyl formate are distilled off from theresultant hydrolysis mixture,

[0009] c) the bottom product from the distillation (b), which comprisesformic acid and water, is extracted in a liquid-liquid extraction withan extractant which principally takes up the formic acid,

[0010] d) the resultant extract phase, comprising formic acid,extractant and some of the water, is subjected to distillation,

[0011] e) the top product obtained in this distillation, which compriseswater and some of the formic acid, is fed back into the lower part ofthe distillation column in step (b),

[0012] f) the bottom product from distillation step (d), whichpredominantly comprises extractant and formic acid, is separated intoanhydrous formic acid and the extractant by distillation, and

[0013] g) the extractant leaving step (f) is fed back into the process.

[0014] In this process, it is particularly advantageous

[0015] h) to carry out distillation steps (b) and (d) in a singlecolumn,

[0016] i) to introduce the water necessary for the hydrolysis in theform of steam into the lower part of the column provided for carryingout step (b),

[0017] k) to employ methyl formate and water in the hydrolysis (a) in amolar ratio of from 1:2 to 1:10, and/or

[0018] l) to employ, as extractant, a carboxamide of the general formulaI

[0019] where the radicals R1 and R2 are alkyl, cycloalkyl, aryl oraralkyl groups, or R1 and R2 jointly, together with the N atom, form ahetero-cyclic 5- or 6-membered ring, and only one of the radicals is anaryl group, and where R3 is hydrogen or a C1-C4-alkyl group.

[0020] Steps (a) to (i) of the above-described process disclosed inEP-B-0 017 866 are explained in greater detail below.

[0021] Step (a)

[0022] The hydrolysis is usually carried out at a temperature in therange from 80 to 150° C.

[0023] Step (b)

[0024] The distillation of the hydrolysis mixture can in principle becarried out at any desired pressure, preferably from 0.5 to 2 bar. Ingeneral, working under atmospheric pressure is advisable. In this case,the temperature at the bottom of the column is about 110° C. and thetemperature at the top of the column is from about 30 to 40° C. Thehydrolysis mixture is advantageously added at a temperature in the rangefrom 80 to 150° C., and the methanol is preferably removed in liquidform at a temperature of from 55 to 65° C. Satisfactory separation ofthe mixture into methyl formate and methanol on the one hand and aqueousformic acid on the other hand is possible even using a distillationcolumn which has 25 theoretical plates (the theoretical number of platesis preferably from 35 to 45). Any design can be used for the columnintended for step (b), but a sieve-plate or packed column isparticularly recommended.

[0025] Step (c)

[0026] The liquid-liquid extraction of the formic acid from its aqueoussolution by means of an extractant is preferably carried out atatmospheric pressure and a temperature of from 60 to 120° C., inparticular from 70 to 90° C., in countercurrent. Depending on the typeof extractant, extraction devices having from 1 to 12 theoreticalseparation stages are generally required. Suitable extraction devicesfor this purpose are in particular liquid-liquid extraction columns. Inmost cases, satisfactory results are achieved using from 4 to 6theoretical separation stages.

[0027] The choice of extractant is not limited. Particularly suitableextractants are carboxamides of the general formula I given above.Extractants of this type are, in particular, N-di-n-butylformamide andin addition N-di-n-butylacetamide, N-methyl-N-2-heptylformamide,N-n-butyl-N-2-ethylhexylformamide, N-n-butyl-N-cyclohexylformamide andN-ethylformanilide, and mixtures of these compounds. Further suitableextractants are, inter alia, diisopropyl ether, methyl isobutyl ketone,ethyl acetate, tributyl phosphate and butanediol formate.

[0028] Step (d)

[0029] The extract phase is separated by distillation in an appropriatedistillation device into a liquid phase, which generally comprisespredominantly formic acid and extractant, and a vapor phasepredominantly comprising water and small amounts of formic acid. This isan extractive distillation. The bottom temperature is preferably from140 to 180° C. A satisfactory separation effect is generally achievedfrom 5 theoretical plates.

[0030] Step (e)

[0031] The formic acid/water mixture is generally recycled in vaporform.

[0032] Steps (f) and (g)

[0033] The distillation device (usually in the form of a column) forcarrying out step (f) is advantageously operated under reducedpressure—from about 50 to 300 mbar and correspondingly low headtemperatures—from about 30 to 60° C.

[0034] Step (h)

[0035] This variant of the process relates to steps (b) and (d). Thedistillation devices for carrying out steps (b) and (d) are arranged inan overall distillation device. The distillation devices here aregenerally in the form of columns.

[0036] Step (i)

[0037] In this step, water required for the hydrolysis is provided inthe form of steam.

[0038] In the process described above, methyl formate is hydrolyzed in ahydrolysis reactor with a molar excess of water. The hydrolysis reactionis preferably carried out as a pure liquid-phase reaction attemperatures from 80 to 150° C. In order to be able to achieve thesetemperatures of the reaction mixture, the hydrolysis must be carried outat superatmospheric pressure—at atmospheric pressure, the reactionmixture would have a boiling point below the temperature range indicatedabove. In the subsequent step, the hydrolysis mixture from thehydrolysis reactor is passed into a distillation column for distillativeseparation. A lower pressure prevails in the latter than comparativelyin the hydrolysis reactor. On introducing the hydrolysis mixture intothe distillation column, the hydrolysis mixture is thus decompressedsuddenly (abrupt drop in pressure). The consequence is vigorous foamingof the hydrolysis mixture introduced into the distillation column. Thefoaming has highly adverse effects on the separation performance of thedistillation internals of the distillation column, since the foaming isassociated with intensive back-mixing of the internal gas and liquidstreams in the distillation column, which preferably run incountercurrent. In addition, the fluid-dynamic working range is greatlyrestricted by the foaming, since the foam causes an unacceptablepressure loss in the column. In order to prevent the foaming, acommercially available antifoam can be added to the distillation column,above the feed point for the hydrolysis mixture. This reduces or greatlyrestricts the foaming, so that the adverse consequences of foamingdescribed above are excluded. Examples of suitable antifoams which canbe employed are silicone oils. The costs of the antifoams have anadverse effect on the economic efficiency of the process. Secondly, theaddition of the commercially available antifoams is accompanied byintroduction of foreign substances into the process, which generallyhave an adverse effect on the product quality. Since the degradationproducts of the antifoams must be eliminated from the process,considerable disposal costs arise in the treatment of the correspondingwaste water.

[0039] It is an object of the present invention to provide a process inwhich foaming is prevented without high costs and an adverse effect onproduct quality occurring. A further aim is to minimize the disposalcosts for the antifoam. The process should be simple and practical tocarry out.

[0040] We have found that this object is achieved by a process forobtaining anhydrous or substantially anhydrous formic acid in which

[0041] i) methyl formate is subjected to hydrolysis,

[0042] ii) methanol and excess methyl formate are distilled off from theresultant hydrolysis mixture,

[0043] iii) the bottom product from distillation ii), comprising formicacid and water, is extracted in a liquid-liquid extraction with anextractant which principally takes up the formic acid, and theextractant employed here is a carboxamide of the general formula I

[0044] where the radicals R1 and R2 are alkyl, cycloalkyl, aryl oraralkyl

[0045] groups, or R1 and R2 jointly, together with the N atom, form aheterocyclic 5- or 6-membered ring, and only one of the radicals is anaryl group, and where R3 is hydrogen or a C1-C4-alkyl group,

[0046] iv) the resultant extract phase, comprising formic acid,extractant and some of the water, is subjected to distillation,

[0047] v) the top product obtained in this distillation, which compriseswater and some of the formic acid, is fed back into the lower part ofthe distillation device in step ii),

[0048] vi) the bottom product from distillation step iv), whichcomprises predominantly extractant and formic acid, is separated bydistillation into anhydrous or substantially anhydrous formic acid andthe extractant, and

[0049] vii) the extractant leaving step vi) is fed back into theprocess,

[0050] which comprises removing a sub-stream of the extractant employedfrom the process and feeding it to the distillation device provided forcarrying out step ii), with the corresponding feed point for thesub-stream of the extractant in the distillation device being above thefeed point of the hydrolysis mixture and below the removal point ofmethanol.

[0051] For the purposes of the present invention, the term“substantially anhydrous formic acid” is taken to mean formic acid whichcontains a maximum of 30%, preferably a maximum of 15%, of water.

[0052] The extractant introduced into the distillation device forcarrying out step ii) prevents foaming. All suitable extractants of thegeneral formula I boil higher than the components of the hydrolysismixture (methyl formate, water, methanol and formic acid). Theextractant introduced into the distillation column for carrying out stepii) thus flows out of the distillation column in liquid form and,together with the aqueous formic acid fed into the extractor, re-entersthe extractant circuit. The advantages of the process according to theinvention are clear. It is no longer necessary to employ an expensiveadditional antifoam which subsequently has to be removed from theprocess again. Costs for purchasing an antifoam and for waste-watertreatment which would be associated with the introduction of an initialantifoam can thus be saved. It is basically advantageous if theintroduction of foreign substances can be avoided in the process. Theextractant as antifoam is thus a component which is already present inthe process anyway—the introduction of an additional antifoam isavoided.

[0053] Preferred extractants are N,N-di-n-butylformamide,N,N-di-n-butylacetamide, N-methyl-N-2-heptylformamide,N-n-butyl-N-2-ethylhexylformamide, N-n-butyl-N-cyclohexylformamideand/or N-ethylformanilide.

[0054] In a preferred embodiment of the invention, the sub-stream of theextractant employed which is fed to the distillation device for carryingout step ii) is taken from the extractant leaving step vi). Inprinciple, however, the extractant for the provision of the sub-streamcan be taken from any desired point of the process. Extractants whichcan be employed are also mixtures which comprise the various extractantsaccording to the invention. Besides extractant, the extractant used asantifoam can also contain other components, in particular formic acid,water, methanol and/or methyl formate. The introduction of additionalantifoams which are not in accordance with the invention is in principlepossible.

[0055] In general, distillation steps ii) and iv) are carried out in asingle distillation device.

[0056] The invention also relates to the use of a carboxamide of thegeneral formula I

[0057] where the radicals R1 and R2 are alkyl, cycloalkyl, aryl oraralkyl groups, or RI and R2 jointly, together with the N atom, form aheterocyclic 5- or 6-membered ring, and only one of the radicals is anaryl group, and where R3 is hydrogen, in the above-described process asantifoam in the distillative separation of the hydrolysis mixturecomprising methyl formate, water, formic acid and methanol, and asextractant for the liquid-liquid extraction of the formic acid.

[0058] The invention also relates to a device for carrying out theprocess explained above, which comprises

[0059] α) a synthesis reactor,

[0060] β) a hydrolysis reactor,

[0061] χ) a distillation device for carrying out step ii),

[0062] δ) a distillation device for carrying out step iv),

[0063] ≮) an extraction device,

[0064] φ) a distillation device for carrying out step vi), and

[0065] γ) a connecting line for feeding a sub-stream of the extractantinto the distillation column provided for carrying out step ii).

[0066] The term “synthesis reactor” is taken to mean a device in whichon the one hand the synthesis of methyl formate is carried out (usuallyin a corresponding reactor) and, if desired, on the other handseparation of the resultant synthesis mixture is carried out (usually ina distillation device downstream of the reactor). The hydrolysis reactoremployed can also be any desired reactor which is suitable for thehydrolysis of methyl formate. The distillation devices are generally inthe form of columns. The extractant device employed is preferably aliquid-liquid extraction column. The connecting line for feeding asub-stream of the extractant into the distillation column proposed forcarrying out step ii) is generally in the form of a tube arrangedbetween the distillation device 2 for carrying out step ii) and anoutlet tube for extractant leaving the distillation device for carryingout step iv).

[0067] In a preferred embodiment of the invention, the distillationdevice for carrying out step ii) and the distillation device forcarrying out step iv) are arranged in a single distillation device. Thelatter is generally in the form of a column.

[0068] The attached drawing shows

[0069] in FIG. 1 and FIG. 2

[0070] diagrams of plants for the preparation of anhydrous orsubstantially anhydrous formic acid in accordance with the prior art,and

[0071] in FIG. 3 and FIG. 4

[0072] diagrams of plants for the preparation of anhydrous orsubstantially anhydrous formic acid in accordance with the processaccording to the invention.

[0073] The reference numerals entered above, below or alongside theconnecting lines or arrows correspond to the components which generallyform the major component of the corresponding stream. Since thecomposition in the streams can vary, these numbers are merely intendedas a guide value. 21 here denotes methyl formate, 22 denotes water, 23denotes formic acid, 24 denotes methanol, 25 denotes extractant and 27denotes carbon monoxide.

[0074] The plants shown diagrammatically in FIG. 1 and FIG. 2 forcarrying out the process in accordance with the prior art and the plantsshown diagrammatically in FIG. 3 and FIG. 4 for carrying out the processaccording to the invention have the common feature that they comprise asynthesis reactor 6, a hydrolysis reactor 1, a distillation device 2 forcarrying out step ii), a distillation device 4 for carrying out stepiv), an extraction device 3 and a distillation device for carrying outstep vi). The distillation devices 2 and 4 may be arranged in a commondistillation device 7.

[0075] In contrast to the prior-art plants shown in FIG. 1 and FIG. 2,the plants shown in FIG. 3 and FIG. 4 for carrying out the processaccording to the invention comprise a connecting line 8 for feeding asub-stream of the extractant into the distillation column 2 proposed forcarrying out step iv). This sub-stream of extractant is taken from theextractant stream leaving the distillation device 5, and isadvantageously cooled. The connecting line 8 runs into the distillationdevice 2 for carrying out step ii) at the feed point 9. This feed point9 for the sub-stream of the extractant is arranged in the distillationdevice above the feed point 11 for the hydrolysis mixture and below theremoval point 10 for methanol. In general, the feed point 9 for theextractant is located from 2 to 40 theoretical stages, in particularfrom 5 to 20 theoretical separation stages, above the feed point 11 forthe hydrolysis mixture.

[0076] The invention will be explained in greater detail below withreference to a working example.

EXAMPLE

[0077] The illustrative experiment according to the invention is carriedout in a plant which is shown diagrammatically in FIG. 4. 5.3 kg/h ofaqueous formic acid are prepared continuously. The distillation device 7used, which is in the form of a pilot-plant column, has a diameter of100 mm and is fitted with 100 bubble-cap plates. The correspondingreactor discharge is 20 kg/h. The hydrolysis mixture is introduced atthe forty-fifth plate, while the antifoam is introduced at thesixty-fifth plate. The column is operated under atmospheric pressure.The extractant employed is N,N-di-n-butylformamide. A 200 g/h extractantstream is fed in 20 plates above the feed 11 for the hydrolysis mixture.This enables foaming to be completely suppressed. This measure enablesthe plates above the feed point 11 for the hydrolysis mixture to achievethe usual efficiency on addition of antifoams of 0.7 theoreticalseparation stages per practical plate. The pressure loss is 2 mbar perplate.

[0078] For Comparison:

[0079] Without addition of an antifoam, vigorous foaming occurs in thedistillation device 2 above the feed point of the hydrolysis mixture 11.The foaming reduces the plate efficiency of the foam-filled plates to0.2 theoretical separation stages per practical plate. The pressure lossis 3.5 mbar per plate.

[0080] The above experiment shows that the extractant employed issuitable as antifoam in the process according to the invention.

We claim:
 1. A process for obtaining anhydrous or substantiallyanhydrous formic acid, in which i) methyl formate is subjected tohydrolysis, ii) methanol and excess methyl formate are distilled offfrom the resultant hydrolysis mixture, iii) the bottom product fromdistillation (ii), comprising formic acid and water, is extracted in aliquid-liquid extraction with an extractant which principally takes upthe formic acid, and the extractant employed here is a carboxamide ofthe general formula I where the radicals R1 and R2 are alkyl,cycloalkyl, aryl or aralkyl

groups, or R1 and R2 jointly, together with the N atom, form aheterocyclic 5- or 6-membered ring, and only one of the radicals is anaryl group, and where R3 is hydrogen or a C1-C4-alkyl group, iv) theresultant extract phase, comprising formic acid, extractant and some ofthe water, is subjected to distillation, v) the top product obtained inthis distillation, which comprises water and some of the formic acid, isfed back into the lower part of the distillation device in step ii), vi)the bottom product from distillation step iv), which comprisespredominantly extractant and formic acid, is separated by distillationinto anhydrous or substantially anhydrous formic acid and theextractant, and vii) the extractant leaving step vi) is fed back intothe process, which comprises removing a sub-stream of the extractantemployed from the process and feeding it to the distillation deviceprovided for carrying out step ii), with the corresponding feed pointfor the sub-stream of the extractant in the distillation device beingabove the feed point of the hydrolysis mixture and below the removalpoint of methanol.
 2. A process as claimed in claim 1, wherein theextractant employed is N,N-di-n-butylformamide, N,N-di-n-butylacetamide,N-methyl-N-2-heptylformamide, N-n-butyl-N-2-ethylhexylformamide,N-n-butyl-N-cyclohexylformamide and/or N-ethylformanilide.
 3. A processas claimed in claim 1 or 2, wherein the sub-stream of the extractantemployed is taken from the extractant leaving step vi).
 4. A process asclaimed in one of claims 1 to 3, wherein distillation steps ii) and iv)are carried out in a single distillation device.
 5. The use of acarboxamide of the general formula I

where the radicals R1 and R2 are alkyl, cycloalkyl, aryl or aralkylgroups, or R1 and R2 jointly, together with the N atom, form aheterocyclic 5- or 6-membered ring, and only one of the radicals is anaryl group, and where R3 is hydrogen or a C1-C4-alkyl group, in theabove-described process as claimed in claims 1 to 4 as antifoam in thedistillative separation of the hydrolysis mixture comprising methylformate, water, formic acid and methanol, and as extractant for theliquid-liquid extraction of the formic acid.
 6. An apparatus forcarrying out a process as claimed in one of claims 1 to 4, comprising α)a synthesis reactor (6), β) a hydrolysis reactor (1), χ) a distillationdevice (2) for carrying out step ii), δ) a distillation device (4) forcarrying out step iv), ≮) an extraction device (3), φ) a distillationdevice (5) for carrying out step vi), and γ) a connecting line (8) forfeeding a sub-stream of the extractant into the distillation column (2)provided for carrying out step ii).
 7. An apparatus as claimed in claim6, wherein the distillation device (2) for carrying out step ii) and thedistillation device (4) for carrying out step iv) are arranged in asingle distillation device.